feat(portal-bff): helmet + env-driven CORS allowlist + double-submit CSRF (#122)

## Summary

Phase-2 security baseline that the `main.ts` placeholder note has been advertising since the auth/session work began. Three independent middlewares + their SPA counterparts, all mounted in a single PR because they only become meaningful together.

### Helmet on the BFF

`helmet()` with three overrides matching our specific shape:

- **HSTS only in production** — dev runs on plain HTTP, HSTS is just noise.
- **`crossOriginResourcePolicy: 'cross-origin'`** — the SPA on its own origin reads JSON from the BFF; the default `same-origin` would block it.
- **CSP disabled in non-production** — the BFF doesn't render HTML, so CSP on JSON responses is mostly inert, but Helmet's default CSP triggers noisy `connect-src` violations in browser devtools that we don't need.

Everything else is Helmet defaults: `X-Frame-Options=SAMEORIGIN`, `X-Content-Type-Options=nosniff`, `Referrer-Policy=no-referrer`, `X-Powered-By` removed, etc.

### CORS allowlist, env-driven

`CORS_ALLOWED_ORIGINS` env (comma-separated) is now **mandatory** at boot. The BFF refuses to start without it via `readCorsAllowlist()` — same boot-time validator family as `assertSessionSecret` etc. The previous hardcoded `http://localhost:4200` fallback is gone; getting CORS wrong silently is the kind of "works in dev, breaks in prod" trap the validator is specifically designed to catch. `X-CSRF-Token` is now in the allowed headers.

### Double-submit CSRF

- BFF mints a 256-bit `csrfToken` at session creation (`/auth/callback`), stored on `req.session.csrfToken` and mirrored to a JS-readable cookie (`__Host-portal_csrf` prod / `portal_csrf` dev). The cookie is the SPA's read-only view; the server-side session is the source of truth.
- `createCsrfMiddleware` (mounted after the session middleware in `main.ts`) compares the `X-CSRF-Token` header with `req.session.csrfToken` using `crypto.timingSafeEqual`. Skips:
  - safe methods (`GET / HEAD / OPTIONS`),
  - anonymous requests (no `req.session.user`),
  - `/api/auth/login` and `/api/auth/callback` (those mint the token themselves).
- Mismatch → `403 {"error":"csrf"}` with a structured Pino warn.
- SPA's `csrfInterceptor` reads the cookie via `document.cookie` and copies its value into `X-CSRF-Token` on every mutating BFF request. The header is omitted on `GET / HEAD / OPTIONS` (BFF skips them anyway) and on non-BFF origins.
- Logout and the absolute-timeout middleware both clear the CSRF cookie alongside the session cookie.

## Notable choices

**Session-bound double-submit, not pure cookie-vs-header.** A naive "compare cookie with header" check is defeated when an attacker can plant a cookie (subdomain takeover, etc.). Comparing the header to the server-side session-stored token instead means the attacker would also need to be the authenticated user — which is what CSRF defense is supposed to prevent in the first place.

**No CSRF for anonymous mutating routes (v1).** None exist today; we don't have an unauthenticated POST endpoint anywhere. Generating a CSRF token for anonymous sessions would conflict with `saveUninitialized: false` on express-session and add complexity we don't need yet. Anonymous public-form CSRF defenses (site-key, captcha) land if and when those routes ship.

**`SameSite=Lax`, not `Strict`, on the CSRF cookie.** Matches the session cookie's policy so the two travel together on the SPA→BFF cross-origin same-site fetch (different ports = different origin, same registrable domain). The double-submit pattern is what gives the protection; `SameSite=Lax` is a belt-and-braces layer.

**`csrfInterceptor` runs after `bffCredentialsInterceptor` and before `bffUnauthorizedInterceptor` in the chain.** Order: credentials first (set `withCredentials`), then CSRF (set the header), then unauthorized handling (catch 401s). Forward order, no surprises.

**`CORS_ALLOWED_ORIGINS` has no localhost fallback.** I considered keeping the fallback for ergonomics but it makes the BFF silently misconfigured if someone forgets the env. The error message points straight at the file to edit.

## Out of scope (next PRs)

- Rate limiting + structured error filter (still in the phase-2 to-do).
- CSP fine-tuning when we have actual HTML pages (portal-shell + portal-admin static serving).
- CSRF token rotation on idle-extension (today the token lives the session's lifetime; refreshing on each request would invalidate in-flight mutations).

## Test plan

- [x] `pnpm nx run-many -t test --projects=portal-bff,feature-auth,portal-shell` clean env → **177 + 28 + 34 = 239/239 pass** (was 144 + 19 + 34 = 197 before; +42 specs across CSRF middleware, CSRF cookie helpers, CORS allowlist parser, csrfInterceptor, and extended auth.controller / absolute-timeout coverage).
- [x] `pnpm nx run-many -t lint build --projects=portal-bff,feature-auth,portal-shell` → clean.
- [x] **CI clean-env repro** (lesson from prior PRs): every env var unset (including new `CORS_ALLOWED_ORIGINS`) → tests still pass. The BFF refuses to boot without `CORS_ALLOWED_ORIGINS`, which is the intended behaviour.
- [x] Prettier-clean.
- [ ] Manual smoke against running BFF:
  - [ ] Sign in → `__Host-portal_csrf` (prod) / `portal_csrf` (dev) cookie set, value matches `audit.events.payload->>actorIdHash`-style traceability via `req.session.csrfToken` in Redis.
  - [ ] Hit a future POST route from the SPA → request carries `X-CSRF-Token`, BFF accepts.
  - [ ] Forge a POST without the header (curl) → 403 `{"error":"csrf"}`.
  - [ ] Sign out → both cookies cleared.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #122

apps/portal-shell/src/app/app.config.ts

@@ -7,8 +7,10 @@ import { provideHttpClient, withFetch, withInterceptors } from '@angular/common/
 import { provideRouter } from '@angular/router';
 import {
   AUTH_BFF_BASE_URL,
+  AUTH_CSRF_COOKIE_NAME,
   bffCredentialsInterceptor,
   bffUnauthorizedInterceptor,
+  csrfInterceptor,
 } from 'feature-auth';
 import { environment } from '../environments/environment';
 import { appRoutes } from './app.routes';
@@ -33,13 +35,18 @@ export const appConfig: ApplicationConfig = {
     //     when a BFF route (other than `/auth/me` itself) answers
     //     401, keeping the SPA's auth state in sync after server-
     //     side session destruction (absolute-timeout, manual revoke).
+    //   - `csrfInterceptor` reads the BFF's CSRF cookie and copies
+    //     the value into the `X-CSRF-Token` header on mutating BFF
+    //     requests (POST / PUT / PATCH / DELETE), per the
+    //     double-submit pattern from ADR-0009.
     provideHttpClient(
       withFetch(),
-      withInterceptors([bffCredentialsInterceptor, bffUnauthorizedInterceptor]),
+      withInterceptors([bffCredentialsInterceptor, csrfInterceptor, bffUnauthorizedInterceptor]),
     ),
-    // `feature-auth` is environment-agnostic and reads the BFF base
-    // URL from this token — provided once per app from `environment.ts`
-    // (per ADR-0018).
+    // `feature-auth` is environment-agnostic and reads its BFF
+    // config from these tokens — provided once per app from
+    // `environment.ts` (per ADR-0018).
     { provide: AUTH_BFF_BASE_URL, useValue: environment.bffApiBaseUrl },
+    { provide: AUTH_CSRF_COOKIE_NAME, useValue: environment.bffCsrfCookieName },
   ],
 };